The invention relates to a hydraulically damping rubber spring device usable as an engine mount for motor vehicles having two end walls lying opposite one another along the axial direction of the spring device and between which are arranged two working chambers. Both working chambers are filled with damping fluid and are separated from each other by a dividing wall in which is arranged a channel connecting the working chambers to one another. The length of said channel is at least equal to the diameter of the dividing wall and the cross-section of the channel is small relative to its length. A rubber-elastic spring element adjoins one of the working chambers and is secured on an end wall. The size of the wall area and the volume of the channel are changeable by complete, or partial, isolation of one longitudinal portion of the channel by intervention from outside.
A rubber spring device of this general type is disclosed in German Patent No. 2,905,090. This rubber spring device, has a multi-turn channel as a connection for the two working chambers with vibrations of low frequency. Vibrations of a high amplitude are subjected to powerful damping by the fluid column, vibrating in the channel, acting as a damper by reason of its internal fiction and its friction against the channel wall. It has now been found that the bandwidth of the frequencies of the vibrations, the damping of which is found to be necessary in many applications, is so great (namely between about 6 and over 20 Hz), that the liquid column in the channel is no longer capable of following the vibrations of the bearing at least in individual frequency ranges and thus, as a result of its low degree of movement, can only generate a low damping force.
The object on which the invention is based is, was to develop a rubber spring device of this general type, so as to achieve better matching of the damping characteristics of the rubber spring device and, in particular, with respect to the frequency and damping force of the actual vibrations acting on the rubber spring device which need to be damped.
This is achieved by having a rotary slide arranged in the channel and transverse to it and with a first longitudinal portion of the channel being open towards one working chamber in one rotary end position of the slide and connected to another longitudinal portion of the channel which opens towards the other working chamber, in the other rotary end position of the valve. Between these rotary end positions, the slide is connected to both longitudinal portions with a continuously changeable passage cross-section.
In the rubber spring device designed according to the invention, with its alterable channel volume, the fluid volumes in the channel can be selected continuously in such a way that the natural frequency of the fluid column in the channel bears a particular relationship to the vibration frequencies imposed from outside. As a result of this, the vibration behavior of the fluid column and the damping force associated therewith can be selected to damper in the desired manner. In particular, in the case of matching the natural frequency of the fluid in the channel to the frequency of the imposed vibration, a high velocity of the fluid in the channel and hence a high damping force is achievable. In addition, in the rubber spring device designed according to the invention, the damping force can be matched to the requirements by reason of the fact that as the fluid volume alters, the internal friction of the fluid alters and as the size of the channel wall area alters, the friction between the latter and the fluid alters.
German Offenlegungsschrift No. 3,404,593 also discloses a rubber spring device in which the damping force can be altered by reason of the fact that a restriction, the size of which can be altered in a small number of steps by intervention from outside, is installed in a short through flow channel between the two fluid chambers. In this arrangement, all that occurs is a more or less pronounced restriction of the quantity of fluid flowing from one chamber to the other. This results, in addition to an alteration of the damping characteristics, in an unfavorable influence on the resiliency properties of the rubber spring device. In particular, high damping forces are not achievable in the case of varying vibration frequencies.
By having two longitudinal portions of the channel arranged one above the other, a length corresponding approximately to the circumference of the dividing wall is possible. Here an alteration of the channel volume and of the size of the channel wall area can be achieved by an alteration of the channel cross-section over at least a considerable length. This alteration of the cross-section can be achieved in a simple manner by means of a diaphragm wall of the channel being enlarged or reduced in response to the application of a pressure or vacuum source to an outer side of the diaphragm.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.